Annette D. George

Miocene cooling in the northern Qilian Shan, northeastern margin of the Tibetan Plateau, revealed by apatite fission-track and vitrinite-reflectance analysis

Apatite fission-track and vitrinite-reflectance data from Phanerozoic rocks of the northern Qilian Shan and Jiuxi basin, at the northeastern margin of the Tibetan Plateau, show a complex thermal history with multiple paleothermal events. Peak paleotemperatures, reached in the middle Cretaceous, suggest that hydrocarbon generation occurred in the basin at that time. Paleozoic-Mesozoic samples show evidence of a middle Tertiary cooling episode between 20 and 10 Ma. This thermal fingerprint is significant because stratigraphic evidence for Miocene exhumation is overwhelmed by spectacular Pliocene-Pleistocene deformation and erosion. We interpret the Miocene cooling as recording initial exhumation of the northern Qilian Shan—earlier than other models have proposed (i.e., after 6– 5 Ma). Miocene cooling is broadly coincident with Cenozoic uplift and exhumation recorded elsewhere in the Tibetan-Tarim region.

Unique marine Permian‐Triassic boundary section from Western Australia

A unique marine Permian‐Triassic boundary section containing rich oil source rocks has been continuously cored in a petroleum borehole from the Perth Basin of Western Australia. Such sequences, which provide a biostratigraphic and environmental record at the time of the largest extinction event of the past 500 million years, are globally rare, and this is the first to be documented in Australia. Throughout geological history there have been periods of global marine anoxia that commonly resulted in the widespread deposition of petroleum source rocks, most notably in the mid‐Cretaceous and Late Jurassic. An apparent paradox is that, previously, source rocks have not been recognised in association with the Permian‐Triassic boundary, despite widespread marine anoxia at this time. The Perth Basin source rocks contain abundant and unusual biomarkers, apparently related to the highly specialised and limited biota that flourished in the aftermath of the end‐Permian extinction event. Local conditions may have favoured source‐rock development, either due to higher productivity resulting from coastal upwelling or through enhanced preservation under strongly anoxic conditions.

Survival brachiopod faunas of the end-Permian mass extinction from the southern Alps (Italy) and South China

Eight brachiopod species in seven genera are described from the Permian–Triassic boundary beds of South China and northern Italy. The brachiopods from northern Italy are described for the first time and include two new species: Orbicoelia dolomitensis Chen and Spirigerella ? teseroi Chen. The Permian affinity of these brachiopods and their stratigraphical position above the extinction horizon demonstrate that they are survivors from the end-Permian mass extinction. The surviving brachiopods from South China, which was located at the eastern margin of the Palaeo-Tethys Ocean, are considerably abundant and diverse and are dominated by geographically widespread generalist elements adapted to a wide variety of environments. They were mostly limited to the Upper Permian to lowest Griesbachian. In contrast, the survivors in northern Italy, which was situated at the western margin of the Palaeo-Tethys, comprise elements ranging from the Carboniferous to Permian or widespread Tethyan genera. These survivors did not occur in the pre-extinction western Tethyan oceans but migrated into this region after the end-Permian extinction event. Disaster taxon Lingula proliferated slightly earlier in western Tethyan oceans than in eastern Tethyan regions following the event. Survival brachiopods from both regions appear to have a generic affinity, although they do not share any species. Both South Chinese and Italian survival faunas support the view that the survival interval is the duration when survivors are dominated by geographically widespread generalist organisms adapted to a wide variety of ecological conditions.

Glacial incursion on a Neoproterozoic carbonate platform in the Kimberley region, Australia

The 80-m-thick Egan Formation preserves sediment deposited during the younger of two episodes of glaciation recorded in the Neoproterozoic succession of the Kimberley region, northwestern Australia. Like many terminal Proterozoic glaciations recorded in Australia and elsewhere, the glacial strata of the Egan Formation are associated with carbonate rocks of likely warm-water affinity, but they are sedimentologically distinct from the marker “cap carbonate” horizons that overlie glacial strata in other Neoproterozoic successions. The carbonate strata comprise a wide range of facies indicative of shallow-water patch reef, shoal, and lagoonal deposition. Detailed facies analysis of the Egan Formation indicates interruption of the carbonate system by glaciation and subsequent resumption of warm-water conditions. This sedimentological analysis allows a reassessment of the regional stratigraphic correlations proposed for the Egan Formation, which is here considered to record a glacial event younger than the widespread Marinoan glaciation of central and South Australia and, therefore, a speculated third global glaciation in terminal Proterozoic time.

The depositional record of the Frasnian/Famennian boundary interval in a fore-reef succession, Canning Basin, Western Australia

Abstract The effects of the global Frasnian/Famennian (F/F) biotic crisis on the shallow-marine reef-builders and associated biota in a laterally extensive late Frasnian reef complex on the Lennard Shelf, northern Canning Basin, are recorded in the well-exposed fore-reef succession (Napier Formation) of the Napier Range. The conodont-defined F/F boundary lies within a distinctive and mappable stratigraphic interval up to 15 m thick which separates highly fossiliferous Frasnian from markedly less fossiliferous Famennian carbonate and siliciclastic lithofacies. Marked lateral facies variation within the interval means that the actual boundary is not represented by a discrete bed or surface. The interval is characterised by a range of lithofacies that record very low sedimentation rates on the slope and reworking and bioturbation of slope strata indicating a condensed interval with negligible sediment input from contemporaneous carbonate platform production. Siliciclastic lithofacies and abundant sponge debris are locally developed. Macrofossil content is generally very low but there is no evidence for anoxic or hypoxic conditions during deposition at a range of palaeowater depths. Allochthonous blocks of stromatoporoid-bearing limestone are absent from the uppermost Frasnian fore-reef strata and stromatoporoid debris is significantly reduced prior to the end of the Frasnian. This suggests that extinction of the stromatoporoid reef margins occurred before the F/F boundary with microbial communities dominating the platform margins prior to the Famennian. Sedimentological and biostratigraphic evidence suggests that a series of sea-level fluctuations, in response to basin tectonism, generated recurring or ongoing environmental stress in the stromatoporoid-dominated reefal communities because of recurring reductions in living space, influxes of siliciclastic sediment supply from the proximal highlands and quite possibly generation of toxic (too saline or too nutrient-rich) lagoonal water. These conditions were potentially superimposed on a global regime of incipient cooling climate leading to widespread glaciation in the Late Carboniferous.

Platform-margin collapse during Famennian reef evolution, Canning Basin, Western Australia

Sedimentological investigations of well-exposed slope strata in the Upper Devonian reef complexes of the northern Canning Basin lead to a sequence-stratigraphic interpretation that differs from others based on predictive models in which coarse carbonate debris deposits are regarded as lowstand deposits. In the upper Frasnian–Famennian Napier Range slope succession, we recognize an important phase of platform-margin collapse during a sea-level highstand in the middle Famennian. At this time the reef-rimmed margin was rapidly prograding, and the platform was producing carbonate grains (notably ooids and peloids) that were being transported by turbidity currents to the slope and basin floor. Periodic collapse of the platform margin led to deposition on the slope of allochthonous blocks of reefal limestone and channelized debris-flow units, intercalated with the ooid-peloidal turbidites and quartzo-feldspathic sandstones. Collapse of the early-cemented margin was probably triggered by gravitational instability caused by oversteepening during rapid progradation and/or tectonic activity, with falling blocks initiating some of the debris flows.

Effects of Middle–Late Permian sea-level changes and mass extinction on the formation of the Tieqiao skeletal mound in the Laibin area, South China

A skeletal mound is described for first time from a Middle–Late Permian succession that contains the Guadalupian–Lopingian (G/L) boundary at the Tieqiao section in the Laibin area, Guangxi Province, South China. The Tieqiao mound grew on a deep-water, south-facing carbonate ramp during the late Middle Permian. The mound features a range of grainstone–packstone to rudstone–floatstone facies together with sponge boundstone facies. Both sponges and algae acted as frame builders which colonised skeletal debris on the ramp and initiated mound growth. Diverse benthic biota comprising mound dwellers and constructors proliferated in the Tieqiao mound complex. The water depth changes recorded in the Tieqiao mound successions reflect the refined trajectory of global sea-level changes during the great regression at the end of the Middle Permian. The timing of sea-level lowstand coincides with disappearance of the Jinogondella granti conodont zone at Tieqiao. The change to sea-level rise pre-dated the G/L boundary. Growth and demise of the mound is interpreted to have been controlled by changes in sea-level. Abrupt disappearance of both body fossils and fossil fragments of leading fossil groups at Tieqiao is interpreted as the result of the G/L mass extinction rather than environmental change. The potential extinction horizon is ∼30 cm above the G/L boundary at Tieqiao and also records a rise in sea-level. The regional fall in sea-level destroyed the Tieqiao mound, but the G/L crisis halted redevelopment of the mound in the earliest Late Permian. Most mound builders suffered severely the Lazarus effect of the G/L mass extinction.

Oxic facies and the Late Devonian mass extinction, Canning Basin, Australia

The close association of anoxic or dysoxic sedimentary rocks and the major Late Devonian (Frasnian–Famennian) mass extinction has focused considerable attention on anoxia as the major cause or as a major factor in a multicausal scenario. The record of the Late Devonian biotic crisis in the well-known reef complexes of northwestern Australia (Canning Basin), in contrast to many localities elsewhere, does not display sedimentological evidence of anoxia through the Frasnian–Famennian boundary interval. Analysis of continuous drill core through this interval has yielded three positive δ 13 C isotopic excursions, only one of which coincides with total organic carbon (TOC) maxima in our data. Multi-element geochemical proxies suggest that TOC maxima preceding positive shifts in δ 13 C most likely resulted from higher productivity caused by nutrient influx from continental weathering, given the close association between TOC maxima and regional relative sea-level falls. Our interpretation supports the view that anoxia was not a fundamental driver of mass extinction and stresses the importance of integrated data sets and understanding regional controls on environmental changes and/or stresses.

Paleokarst in an Upper Devonian Reef Complex of the Canning Basin, Western Australia

ABSTRACT Evidence for subaerial exposure of an early Famennian platform is recorded within the fore-reef slope succession in the Napier Range reef complex of the northern Canning Basin. This observation supports recent sequence-stratigraphic interpretations of fluctuations in relative sea level throughout reef evolution in this basin, and contrasts with previous outcrop-based interpretations that there were no sea-level falls except at the Frasnian-Famennian boundary. Remnants of the paleokarst surface are preserved in allochthonous reef-flat blocks that are temporally and spatially related to lenticular siliciclastic units. Subaerial exposure of the platform margin during a third-order sea-level fall in the early Famennian led to the formation of karst cavities on the reef flat and collapse of parts of the reef margin. During this time, siliciclastic sands were fed to the slope, where they were deposited as a series of aprons along the margin. In the early stages of the ensuing transgression, renewed marine conditions enabled cyanobacteria to colonize the cavities. As sea level continued to rise, siliciclastic sands were reworked across the platform, filling the encrusted cavities. The reef-flat blocks were dislodged during the late transgression n response to instability that may have been caused by triggers such as rising pore-fluid pressures and/or tectonic activity. They were emplaced on the siliciclastic slope aprons towards the end of sand deposition as sea level on the platform continued to rise and carbonate production resumed.

Syndepositional fault control on lower Frasnian platform evolution, Lennard Shelf, Canning Basin, Australia

Syndepositional faulting was a major control on internal platform stratigraphy in a Frasnian reef complex on the southeastern Lennard Shelf, northern Canning Basin, Australia. By combining platform (mostly backreef) facies distributions with key stratal surfaces and biostratigraphic data, we have developed a temporal framework for the Hull Range area. Platform evolution was controlled by normal faults via an initial tilt block geometry and subsequent differential subsidence and accommodation across the platform. Three third-order, flooding surface–bounded platform phases are recorded. Basal shallow marine siliciclastic facies were deposited in topographic lows adjacent to the hanging wall and close to internal faults. Early carbonate deposition in dominantly deep subtidal environments suggests an overall ramp-style setting that deepened toward the southeast. This phase is capped by a sequence boundary represented by multiple paleokarst surfaces in the northwest that pass laterally into a major flooding surface to the southeast, above which the platform expanded toward the Mount Elma–Painted Rocks fault system. The overall stacking pattern above the sequence boundary–flooding surface is aggradational to progradational with higher-frequency shoaling trends, and locally developed exposure surfaces, related to higher-order relative sea-level changes. This second phase of platform growth ended with major flooding and a pronounced backstep of the leeward margin.